my keyboard mappings for Ducky Mini with HJKL Arrow keys like vim
private.xml lives here
/Users/john/Library/Application Support/Karabiner/private.xml
I was able to get linux keyboard mappings to work with GalliumOS on a Acer chromebook.
So this should also work for xubuntu, and probably ubuntu also.
diff the your /usr/share/X11/xkb/symbols/us and the copy of that file here.
port those changed into your 'us' file.
Then delete any files in /var/lib/xkb/server-*
reboot
after reboot, you can run: xmodmap ./dot_Xmodemap
and you should be all set